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VY CMa: Band 9, high angular resolution test observation with band-to-band phase referencing taken in High Frequency Long Baseline campaign 2017

Science Target Overview

VY CMa is located at a distance of 1.17 kpc (Zhang et al. 2012). It is an oxygen-rich red supergiant (RSG) with a variable but exceptionally high mass-loss rate. The high mass-loss rate (up to 10-3 solar mass/yr) provides bright spectral line and continuum emissions within a few arcseconds, making it one of the best targets for the image fidelity check by comparison with the previous SV in Band 9 in with the maximum baseline of 2.7 km (ADS/JAO.ALMA #2011.0.00011.SV, hereafter LBC-2013-SV) not only for the continuum emission but also for the 658.00655 GHz H2O maser (Richards et al. 2014).

 

ALMA Data Overview

On 2017 November 3,VY CMa was observed in Band 9 as a test observation in High Frequency Long Baseline Campaign (Asaki et al. 2020). This test was conducted for the feasibility study of high frequency band-to-band (B2B) phase referencing. The scan lengths for VY CMa and the phase calibrator QSO J0725-260, at 1.1 deg away from VY CMa, were 62 and 12 s, respectively (switching cycle time of 82 s). The total observing time was 47 minutes, and the on-source time for VY CMa was 344 s. The PWV was 0.56 mm. The LO1 frequencies for the target and the phase calibrator are 669 (Band 9) and 149 (Band 4) GHz, respectively. The correlator was configured to have eight SPWs in Band 9 with a bandwidth of 1.875 GHz each using the 90 deg phase switching. One of the SPWs covers the 658 GHz H2O maser with the frequency resolution of 15.625 kHz. A bright QSO, J0522-3627, 28 deg away from the target, was used as the Differential Gain calibration, as well as for bandpass and flux-scale calibration.

The line-free continuum emission was imaged using the averaged eight spectral windows with Briggs weighting (robust = 2), achieving the synthesized beam size is 12 × 11 mas, and the image RMS noise is 1.5 mJy/beam. In the initial map, there is a bright and compact component (VY CMa) that cannot be resolved even with the longest baselines, so that the self-calibration can be applied to the data. In order to obtain the visibility data free from the atmospheric and instrumental phase errors, phase and amplitude self-calibrations were performed for the line-free continuum channels with the solution interval of 16 s to remove residual phase offsets between the spectral windows. The self-calibration solutions were then applied to all target data, and the bright maser peak, which has a higher S/N, was used for further self-calibration, making and applying the solutions with an interval of 4 s to all data. Improvements were seen on the continuum peak, which increased from 42 to 135 mJy/beam, while the image RMS noise was not changed. This indicates that stochastic phase errors remained even after B2B phase referencing, which gave an image coherence loss of ~70%. The rather long switching cycle time in Band 9 may cause a significant coherence loss due to the atmosphere. The self-calibrated continuum image shows that VY CMa can be resolved almost into a point source with minor diffuse emission. Note that O’Gorman et al. (2015) estimated a stellar contribution of 111 -124 mJy at around 658 GHz based on the analytical stellar properties.

A bright submm H2O maser cube with a velocity resolution of 0.5 km/s was generated for the 128 velocity channels. For the H2O maser cube, we applied Briggs weighting (robust = 0.5) to achieve a higher angular resolution because astronomical maser emissions are thought to be very compact (typically 1 au, or 0.9 mas at a distance of 1.17 kpc), so that the maser emission is compact even with a narrower synthesized beam: the resultant synthesized beam size is 10 × 8 mas.

A fait submm SiO maser cube with a velocity resolution of 2 km/s was generated as well for the 16 velocity channels with Briggs weighting (robust = 1.5). It can be seen that the SiO maser emission at 676 GHz is located at inner area of the above H2O maser emission.

 

Using the data for publication

The following statement should be included in the acknowledgement of papers using the dataset listed above:

"This paper makes use of the following ALMA data: ADS/JAO.ALMA#2011.0.00006.E. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada) and MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ."

 

Obtaining the Data

The data products are contained in the following downloadable files:

Readme
Uncalibrated data
Calibrated data
Data reduction scripts
Reference images